Every sturdy scaffold tower starts at ground level. On uneven sites, muddy yards, or sloped surfaces, it is the base jack scaffolding that carries the first load and sets the platform out perfectly level. If you are planning concrete formwork, ringlock towers, or shoring for a heavy industrial job, getting the base jack specifications right is not a detailit is the difference between a safe, stable structure and a costly, risky one. This guide breaks down the specifications that matter for heavy-duty use so you can specify, source, and install with confidence.
A base jack, also called a screw jack or leveling jack, is an adjustable steel support placed under the vertical standards of a scaffold or shoring system. It is made of three parts: a threaded spindle that slides inside the vertical tube, a heavy base plate that distributes the load, and a rotation collar nut that moves the spindle up or down. By turning the nut, you raise or lower each standard independently so the whole structure can be brought to a true, level bearing even on rough ground.
For heavy-duty applications, the base jack does far more than level the platform. It transfers the full vertical load of the structure and the working load into the ground through the base plate, preventing the scaffold from sinking, rocking, or punching into soft soil. Specifying the correct adjustable base jack is therefore a structural decision, not just an ordering decision.
When you compare products, focus on these seven specifications. They determine whether a base jack can safely handle the loads and conditions on your site.
The threaded spindle diameter must suit the tube it seats inside. Common diameters for scaffolding work are 30 mm, 32 mm, 34 mm, and 38 mm, with larger 48 mm spindles used where the bracing and standards demand a stiffer bearing. A thicker spindle gives more stiffness and reduces the risk of bending under heavy eccentric loads, so choose the largest diameter your vertical tube will accept.
Base jacks are commonly supplied in overall lengths of 450 mm, 600 mm, 750 mm, and 900 mm. As a working rule, the safe adjustment length should not exceed about two-thirds of the total spindle length. This keeps enough of the spindle engaged inside the vertical tube to resist buckling. In practice, a 600 mm base jack with a 150 mm safety overlap leaves roughly 450 mm of useable extension.
The base plate is what stops the whole structure from sinking into the ground. Common, duty-capable sizes are 120 x 120 mm and 150 x 150 mm, with plate thicknesses from 5 mm to 8 mm. For soft, gravelly, or wet ground, a larger and thicker plate spreads the load over a wider footprint. Pair the base jack with a firm timber or steel sole plate when the bearing surface is not load-bearing on its own.
The raw material determines strength and fatigue life. Carbon steel grades such as Q235 are the standard choice for general access work, while Q355 offers higher yield strength for heavy-duty shoring and concrete formwork where dense load concentrations are expected. Confirm the steel grade with the mill certificate before you order large volumes.
Two thread profiles are common. Square or acme (trapezoidal) threads are preferred for heavy loads because the wider thread flank resists stripping and keeps the nut turning smoothly under load. Cold-rolled threads are harder and last longer than cut threads, and they hold up better when cement slurry or grit splashes onto the screw on a working site.
The finish controls how long the base jack survives on site. Hot-dip galvanized (HDG) coating is the rugged option for outdoor, coastal, and marine jobs because it forms a thick, metallurgically bonded zinc layer that protects the thread and the inside of the hollow spindle from rust. Electro-galvanized and painted finishes suit drier, short-term assignments.
Heavy-duty base jacks are rated by safe working load, so ask your supplier for the load capacity and the standard it is tested against. Reputable components are manufactured and verified to recognized scaffold standards such as BS 1139 and EN 74. Request the load test report and material certificate rather than relying on a brochure figure.
Base jacks come in two spindle forms, and the choice changes how the product behaves under heavy load.
Solid base jacks use a round steel bar spindle. They offer the highest stiffness and load-carrying capacity, which makes them the default for heavy-duty shoring, slab support, and concrete formwork where loads are concentrated. Their weight is the trade-offthey are heavier to lift and to ship.
Hollow base jacks use a thick-wall steel pipe spindle. They are lighter and easier to handle and transport, which reduces shipping cost and on-site labor. A well-made hollow base jack still carries substantial load, but the overall capacity is lower than an equivalent solid spindle. Hollow jacks are a sound pick for standard modular access towers and lighter duty cycles.
For genuinely heavy-duty work that stays in place for a long time, solid spindles are usually the safer bet; for frequent assembly, relocation, and lighter workloads, hollow spindles win on handling.
A base jack fails at three points: the weld, the thread, and the metal itself. Inspect all three before you accept a delivery.
The weld between the base plate and the spindle must be fully penetrated, free of porosity and cracks, and set perpendicular so the plate sits flat. A tilting plate guarantees uneven load distribution from the first lift.
The collar nut should rotate and tighten smoothly along the full thread without jamming or wobbling. Rough spots or binding threads on a new unit are a warning about the batch quality and the cold-rolling process.
Request a mill certificate matching the steel grade you specified, together with the load capacity test report. This paperwork turns a heavy-duty label into a verifiable spec.
Even a correctly specified base jack is only as safe as its set-up. Keep these rules in mind on every lift:
- Maintain a minimum safety overlap of roughly 150 mm inside the vertical tube so the spindle cannot buckle under load.
- Keep the usable extension to no more than two-thirds of the spindle length, and always use a firm, level sole plate on soft ground.
- Level the base jack before installing the next scaffold lift; never use the scaffold to correct a leveling error after assembly.
- Re-check leveling after any ground movement, rainfall, or loading change during the job.
Heavy-duty base jacks are the quiet workhorses of every scaffold project, yet their specifications are easy to get wrong. Working with a supplier that manufactures against international standards, offers both hollow base jack and solid spindle options, and delivers verified certificates saves you rework, downtime, and risk. Bythai Scaffolding supplies complete scaffolding systems by design, pairing base jacks with ringlock, frame, and tube and clamp systems so every component matches the load you are building fornot just the price you are paying.
If you are specifying base jacks for a heavy-duty project and want a product that carries the right load rating, plating, and paper trail, talk to our team with your tube sizes and site conditions and we will help you select the right unit for every standard.